1. Field of the Invention
The present invention relates to a semiconductor laser device having an AlGaN cladding layer. In particular, the present invention relates to an AlInGaN semiconductor laser device having an AlGaN cladding layer.
2. Description of the Related Art
Currently, light emitting diodes (LEDs) and semiconductor laser devices made of AlInGaN materials are receiving attention as light sources which emit light in the short wavelength range up to 600 nm. For example, S. Nakamura et al. (xe2x80x9cHigh-Brightness InGaN Blue, Green and Yellow Light-Emitting Diodes with Quantum Well Structures,xe2x80x9d Japanese Journal of Applied Physics, Vol. 34(1995), Part 2, No. 7A, L797-L799) report that high-brightness LEDs being made of AlInGaN materials and emitting light in the blue and green wavelength ranges have outstanding properties. At present, the blue and green high-brightness LEDs made of AlInGaN materials are widely used as light sources in signals and outdoor display apparatuses. In addition, semiconductor laser devices which emit light in the wavelength range of 390 to 410 nm are proceeding toward practical utilization. The shortest wavelength of laser light emitted from a semiconductor laser device which is available in the year 2000 is 630 nm. As the 390 to 410 nm laser is capable of producing a much smaller light spot than that produced by the 630 nm laser, it is expected to be used to increase recording density of optical disks. Further, short-wavelength light sources which emit light in the short wavelength range up to 450 nm are regarded as a promising candidate for light sources in digital-image forming apparatuses used in applications (such as printing) in which sensitized material having high sensitivity in the short wavelength range is used. When semiconductor laser devices are used as the light sources in the above applications, the semiconductor laser devices are required to emit a high-quality Gaussian light beam in a single oscillation mode.
However, in order to generate a high-quality Gaussian light beam in a single oscillation mode, the following problem should be overcome.
In order to satisfactorily confine light in the thickness direction of an optical waveguide structure, it is desirable that the thickness of each cladding layer including an AlGaN layer is at least 1 micrometer as in the cases of semiconductor laser devices made of AlGaAs, AlGaInP, or InGaAsP materials. However, since the AlGaN layers have great strain, cracks are produced (i.e., the crystals are broken) when the thicknesses of the AlGaN layers are increased. For example, M. Onomura et al. (xe2x80x9cAnalysis of Transverse Modes of Nitride-based Laser Diodes,xe2x80x9d IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 3, 1999, pp. 765-770) report that when the thickness of an AlGaN cladding layer is increased to about 0.6 micrometers or more, a crack is produced. Therefore, the confinement of laser light becomes insufficient, propagation loss occurs, and the beam radiation pattern is deformed.
An object of the present invention is to provide a reliable semiconductor laser device which prevents production of a crack and leakage of light out of optical waveguide layers, and outputs a high-quality Gaussian laser beam having a uniform optical density.
According to the present invention, there is provided a semiconductor laser device comprising an optical guide region and first and second cladding layers. The optical guide region includes an active layer and first and second optical waveguide layers respectively formed above and under the active layer. The first and second cladding layers are respectively formed above and under the optical guide region. Each of the first and second cladding layers includes at least one AlGaN layer. The thickness tg of the optical guide layer and the thicknesses tc1 and tc2 of the first and second cladding layers satisfy a condition that
xe2x88x920.25 tg+500xe2x89xa6tcxe2x89xa6500 and 400xe2x89xa6 tg when tgxe2x89xa61600, and
100xe2x89xa6tcxe2x89xa6500 when tg greater than 1600,
where tc (nm) represents each of the thicknesses tc1 and tc2 of the first and second cladding layers.
According to the above construction, the leakage of light from the optical guide region into the cladding layers can be reduced, and the propagation loss can also be reduced to at most 1 cmxe2x88x921. Therefore, a high-quality Gaussian laser beam having a uniform optical density can be obtained.
In addition, since the leakage of light from the optical guide region into the cladding layers is reduced, the driving current can also be reduced.
Further, since the differential efficiency is not decreased, the output power can be increased.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) and (ii).
(i) The first thickness tg further satisfies a condition that tgxe2x89xa62000.
(ii) The at least one AlGaN layer is made of AlxGa1xe2x88x92xN, where xxe2x89xa70.1.